Light speed, and that’s final
On any day of the week, two ordinary objects striking each other from opposite directions will experience an impact speed the sum of their individual speeds. Photons in two beams of light, on the other hand, will not meet at twice the speed of light, but at the speed of light; the speed of any particle relative to another particle never exceeds c.

Einstein’s special theory of relativity accounts for some of the fundamental deviations from classical physics – the relativistic relationship between space and time, and the breakdown of Newtonian mechanics at high speeds. The classic expectation was for time and space to measure consistently, regardless of perspective or the speed of the inertial reference frame; and Newton’s laws were thought to hold true at all velocities. The actuality is otherwise, although the discrepancy isn’t often a factor in conventional science and engineering. But as an object approaches light speed, not only do classical or Newtonian mechanics fall short, drastic distortions in distance and duration occur; the difference in perceived reality between the travelling object and a stationary observer becomes astronomical.

Up in the air
Special relativity is for all purposes a law rather than a theory, one that has to be considered for accurate measurements in conjunction with high speed and large distances. The “theory” has been fairly proven by such obvious means as flying highprecision atomic clocks on jetliners for hours at a time – upon comparison with elapsed “ground” time, the clocks consistently show a time lag from being in motion.

Unachievable, see?
The speed of light (c) is the limit. As a particle approaches the speed of light, its energy approaches infinity, and therefore it never quite gets there. Classical physics would have us believe otherwise.